Into this situation comes the voice of God. The Lord personally calls Samuel by name, and effectively commissions him as a prophet by giving him a message to deliver to Eli. (The role of Eli in this process is somewhat ambivalent, in keeping with the presentation of his character throughout the book). There is no doubt in anyone's mind that this is God speaking: "the Lord was with him, and let none of his words fall to the ground. And all Israel... knew that Samuel was established as a prophet of the Lord".
What interests me most in this chapter is the end result: "the Lord appeared again at Shiloh, for the Lord revealed himself to Samuel at Shiloh by the word of the Lord". Note that no phsyical apparition is in view here - the call narrative rules that out. By speaking to Samuel, God appears to him. By giving him his word, God reveals himself. Now Samuel knows the Lord.
Of course, Samuel had a great deal of information about the Lord at the beginning of this chapter. He had access to reports of God's past revelation of himself to Israel, he had knowledge of God's prescribed worship (probably - although he is reported to be sleeping where the Ark is kept!), and he doubtless had instruction from God's priest. But until the Lord spoke to him personally, God was not revealed to him. Facts and reports did not constitute personal revelation.